A DNA–protein complex in the nucleus of eukaryotic cells. Chromosomes are linear (usually) or circular structures containing deoxyribonucleic acid (DNA) complexed with histone and nonhistone proteins, a centromere, and a telomere at each end, if linear. These threadlike, tightly packed structures are seen in animals, plants, and other eukaryotes during mitotic and meiotic cell divisions (Fig. 1). The single DNA molecule in each chromosome carries a unique complement of linearly arranged genes. Chromosomes were so named because, after staining, nineteenth-century light microscopists saw chromosomes as colored bodies in cells. The combination of DNA and proteins in chromosomes is called chromatin. Collectively, the DNA from all the chromosomes in a nucleus is the hereditary blueprint for the species. Eukaryotes (plants, animals, fungi, and protists) usually have one nucleus in each cell. A nucleus is a double membrane-bound compartment in which chromosomes are located in interphase between cell divisions. In contrast, prokaryotes, such as bacteria and their allies [including mitochondria and chloroplasts (cell plastids)], do not have true chromosomes or nuclei because prokaryotes do not confine their small, single circle of DNA in a membrane-bound compartment. See also: Cell division; Deoxyribonucleic acid (DNA); Eukaryotae; Gene; Histone; Meiosis; Mitosis; Protein

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